Semiconductor light emitting diode (LED) devices have been made since the early 1960s and currently are manufactured for usage in a wide range of consumer and commercial applications. The layers comprising the LEDs are based on crystalline semiconductor materials that require ultra-high vacuum techniques for their growth, such as, metal organic chemical vapor deposition (MOCVD). In addition, the layers typically need to be grown on nearly lattice-matched substrates in order to form defect-free layers. These crystalline-based inorganic LEDs have the advantages of high brightness (due to layers with high conductivities), long lifetimes, good environmental stability, and good external quantum efficiencies. The usage of crystalline semiconductor layers that results in all of these advantages, also leads to a number of disadvantages. The dominant ones are high manufacturing costs, difficulty in combining multi-color output from the same chip, and the need for high cost and rigid substrates.
In the mid 1980s, organic light emitting diodes (OLED) were invented (Tang et al, Appl. Phys. Lett. 51, 913 (1987)) based on the usage of small molecular weight molecules. In the early 1990s, polymeric LEDs were invented (Burroughes et al., Nature 347, 539 (1990)). In the ensuing 15 years organic based LED displays have been brought out into the marketplace and there has been great improvements in device lifetime, efficiency, and brightness. For example, devices containing phosphorescent emitters have external quantum efficiencies as high as 19%; whereas, device lifetimes are routinely reported at many tens of thousands of hours. In comparison to crystalline-based inorganic LEDs, OLEDs have much reduced brightness (mainly due to small carrier mobilities), shorter lifetimes, and require expensive encapsulation for device operation. On the other hand, OLEDs enjoy the benefits of potentially lower manufacturing cost, the ability to emit multi-colors from the same device, and the promise of flexible displays if the encapsulation issue can be resolved.
To improve the performance of OLEDs, in the later 1990s, OLED devices containing mixed emitters of organics and quantum dots were introduced (Matoussi et al., J. Appl. Phys. 83, 7965 (1998)). The virtue of adding quantum dots to the emitter layers is that the color gamut of the device could be enhanced; red, green, and blue emission could be obtained by simply varying the quantum dot particle size; and the manufacturing cost could be reduced. Because of problems, such as, aggregation of the quantum dots in the emitter layer, the efficiency of these devices was rather low in comparison with typical OLED devices. The efficiency was even poorer when a neat film of quantum dots was used as the emitter layer (Hikmet et al., J. Appl. Phys. 93, 3509 (2003)). The poor efficiency was attributed to the insulating nature of the quantum dot layer. Later the efficiency was boosted (to ˜1.5 cd/A) upon depositing a monolayer film of quantum dots between organic hole and electron transport layers (Coe et al., Nature 420, 800 (2002)). It was stated that luminescence from the quantum dots occurred mainly as a result of Forster energy transfer from excitons on the organic molecules (electron-hole recombination occurs on the organic molecules). Regardless of any future improvements in efficiency, these hybrid devices still suffer from all of the drawbacks associated with pure OLED devices.
Recently, a mainly all-inorganic LED was constructed (Mueller et al., Nano Letters 5, 1039 (2005)) by sandwiching a monolayer thick core/shell CdSe/ZnS quantum dot layer between vacuum deposited (MOCVD) n- and p-GaN layers. The resulting device had a poor external quantum efficiency of 0.001 to 0.01%. Part of that problem could be associated with the organic ligands of trioctylphosphine oxide (TOPO) and trioctylphosphine (TOP) that were reported to be present post growth. These organic ligands are insulators and would result in poor electron and hole injection onto the quantum dots. In addition, the remainder of the structure is costly to manufacture due to the usage of electron and hole semiconducting layers grown by high vacuum techniques, and the usage of sapphire substrates.
Alivisatos et al., U.S. Pat. No. 5,537,000, the entire disclosure of which is incorporated herein by reference, describe an electroluminescent device wherein the light-emitting layer includes semiconductor nanocrystals (quantum dots) that are formed into one or more monolayers. The monolayers are formed, for example, by use of multifunctional linking agents which cause the nanocrystals to bond to the linking agent which, in turn, bonds to the substrate or support, to form the first monolayer. Linking agents can then be used again to bond the first monolayer of nanocrystals to a subsequent nanocrystal monolayer. Useful linking agents include difunctional thiols, and linking agents containing a thiol group and a carboxyl group. Organic linking agents are poor conductors of electrons and holes. Thus, Alivisatos et al. does not provide a sufficient means of conducting carriers into the light-emitting layer and further into the quantum dots in order to achieve efficient light emission.
Su et al., U.S. Pat. No. 6,838,816, the entire disclosure of which is incorporated herein by reference, describes a method for fabricating a light-emitting source using luminescent colloid nanoparticles (quantum dots). The colloid nanoparticles can be dispersed homogeneously in liquid that can be coated on a substrate to from a light-emitting layer. In certain cases, SiO2 particles are added to the layer of colloidal nanoparticles and the layer is annealed. Adding these particles aids in sealing the layer and protecting the quantum dots from interaction with environmental oxygen. The light-emitting layer is incorporated into an LED, however, the light-emission obtained is not sufficiently high since the method of Su et al. also does not provide a good means for conduction of electrons and hole within the light-emitting layer and into the quantum dot emitters.
Kahen, U.S. Patent Application Publication No. 2007/0057263, the entire disclosure of which is incorporated herein by reference, describes an inorganic light-emitting layer formed from a colloidal dispersion of core/shell quantum dot emitters and semiconductor nanoparticles. Core/shell quantum dots were prepared with non-volatile ligands that can withstand the temperatures used in their synthesis. The quantum dots were separated from the solvent used in the synthesis and the non-volatile ligands were exchanged for volatile ligands. A new colloidal dispersion was prepared by mixing a dispersion of core/shell quantum dots having volatile ligands and a dispersion of semiconductor nanoparticles; this new dispersion was applied to a substrate and annealed. Annealing performs two functions: it removes the volatile ligands and transforms the nanoparticles into a semiconductor matrix. The semiconductor matrix provides a conductive path that can facilitate the injection of a hole or an election into the light-emitting layer and into the core of a quantum dot; subsequent recombination of holes and electrons provides efficient light emission.
Ligand exchange requires separation of quantum dots from a solvent, which can be difficult, since the quantum dots are extremely small. For example, attempts to separate quantum dots by centrifugation of a colloid dispersion may precipitate only a fraction of the dots, even after prolonged times. In addition, if very high centrifugation speeds are employed, it can be very difficult to re-disperse the resulting tightly-packed quantum dot precipitate.
Accordingly, it would be highly beneficial to have a high yield process for forming a colloidal dispersion containing quantum dot emitters for use in coating a light-emitting layer. Furthermore, it would be beneficial to construct an all inorganic LED using this colloidal dispersion and low cost deposition techniques. Additionally, it is desirable to have an all inorganic LED whose individual layers have good conductivity performance. The resulting LED would combine many of the desired attributes of crystalline LEDs and organic LEDs.